This is a small species having a dark silver-grey appearance on its upperside, except for (in general) 2 small white bands on the abdomen. The underside is much brighter with a light silvery ground colouration. The palps are very strikingly pure white.
P. affinis responds very well to the pheromone developed for S. myopaeformis and is attracted in the afternoon, typically from around 15h till 18h. But this is, as always, very dependent on the climatological circumstances of the moment.

The dark-brown eggs are elliptically shaped.

The larva is off-white and has a brown head and the prothoracic shield is only slightly sclerotised and thus very pale. As in all Sesiidae, the prolegs have a double wreath of small hooks.

The caterpillar constructs a rather short gallery in the root of the hostplant.

The caterpillar does not construct a real cocoon but a with silk lined pupation chamber. The pupa itself is coloured bright yellowish brown.

The caterpillar lives in the rootstock of its the hostplant and produces a fair amount of frass which is yellow-brown and much paler than with most other Sesiidae. It overwinters as a small larva and, following spring, continues feeding on the sap in the root of the plant. Because P. affinis reaches the northern border of its distribution in our regions, the development takes most likely 2 years while in more southern –and thus warmer– countries it takes only one year.
After hibernation, the larva prepares an exit hole that is not visible from the outside or consists of a small dome of spun plant-particles. It than pupates without constructing a cocoon. In general, the imagines hatch in early morning.

The larva bores into the rootstock of mainly Helianthemum nummularium but there are also observations in H. apenninum. Also Fumana procumbens is a known hostplant. In literature also other Helianthemum species are mentioned.

This species is restricted to xerothermic calcareous slopes where the hostplant grows.